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Halley's Comet

Halley's Comet streaks across the sky in this photograph from May 25, 1910. Earth passed through Halley's tail that month, prompting fears that toxic gases in the tail would infiltrate the atmosphere. [ESA/Courtesy of D.A. Klinglesmith, J. Rahe]

It looked down upon the fall of Jerusalem, the Battle of Hastings, and the fall of the Alamo. It was there for Mark Twain's birth -- and his death. And it provided confirmation that the laws of gravity apply to all the bodies in the heavens.

This witness to history -- and history maker -- is Halley's Comet. Through the ages, it's inspired fear and wonder -- often at the same time. A good example is its appearance 100 years ago this month. It drew skywatchers to rooftops and comet parties. But Earth's passage through its long tail caused many others to cower in fear. More about that tomorrow.

The comet is a "dirty snowball" -- an oddly shaped chunk of ice mixed with rock and dirt. It orbits the Sun once every 76 years or so. On each approach to the Sun, it sprouts a long, glowing tail.

The comet was first recorded by Chinese astronomers in 240 B.C. During many of its following encounters, it was seen as an omen of death and destruction. Yet no one knew that these encounters were all repeat appearances of a single object -- each comet was considered a one-time event.

In the 17th century, though, British astronomer Edmund Halley changed that. He plotted the comet's orbit using the laws of gravity recently devised by his friend, Isaac Newton. And he predicted that the comet would return to view again around the end of 1758. When it did, astronomers named the comet in Halley's honor -- assuring Halley a bit of immortality.

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